STUDIES ON AN 

 ICHTHYOSPORIDIUM INFECTION IN FISH: 

 TRANSMISSION AND HOST SPECIFICITY 



The causative agent of an epizootic 

 among rainbow trout (S almo gairdneri) was 

 described in an earlier paper by Rucker and 

 Gustafson (1953). ITiis protist of uncertain 

 classification was assigned to the genus 

 Ichthyosporidlum , Caullery and Mesnill,( 1905) . 

 The specific identity is confused by the mulip- 

 licity of species assigned to the genus and to 

 the synonymous genus, Ichthyophonus . The 

 organism we have here appears to be most 

 generally like Ichthyosporidlum hoferi 

 (Plehn and Mulsow 1911), as described by 

 Ellis (1930), and the similar form described 

 by Robertson (1909). It differs from the or- 

 ganism studied by Fish (1934), and Sinder- 

 mann and Scattergood (1954), in the sites of 

 the lesions, the host range, and the character- 

 istic "hyphal" forms. The serious nature of 

 this disease, as previously demonstrated, 

 directed our observations and experiments 

 towards information relating to transmission, 

 host specificity, and prognosis Some in- 

 formation on the transmission and host range 

 of other closely related organisms has been 

 established. An infection was produced in 

 flounders by Fish (1934) through feeding 

 diseased herring; in Salmonidae by Neres- 

 heimer and Clodi (1914) by feeding paiasitized 

 tissue; and in tench, carp, and perch by 

 Pettit (postscript, Neresheimer and Clodi 

 1914) through feeding and also by letting the 

 fish live together with infected salmonoids. 



cases, these organs showed overwhelming 

 infections with large numbers of spheres 

 (fig. 1) and "hyphal" stages (fig. 2) which 

 were most common in chronic, well- 

 established infections . The organism could 

 also be found in the spleen, heart, and 

 peritoneal surface, and occasionally in ad- 

 jacent body musculature, wall of the stomach, 

 and other portions of the gut. These sites 

 were less favorable than the kidney and 

 liver in determining whether or not a fish was 

 infected. Skin lesions commonly produced 

 by the organism described by Sindermann 

 and Scattergood (1954) were not seen. No 

 brain infections were observed, which is 

 unlike the findings of Plehn and Mulsow (1911). 



Material for parenteral inoculation was 

 prepared by blending fresh, infected vis- 

 cera with an equal amount of saline in a 

 blender for a few seconds . Material for 

 feeding was prepared by mincing fresh 

 infected viscera composed mostly of kidney 

 and liver, with knives . Fish were main- 

 tained in individual troughs in running fresh 

 water, where the temperatures ran^d from 

 5Q? te 60' F. 



TRANSMISSION EXPERIMENTS 



The information on the transmission ex- 

 periments IS summarized in Table 1 . 



MATERIALS AND METHODS 



The infective agent was from the same 

 stock of fish as that recorded in the previous 

 paper by Rucker and Gustafson (1953) and 

 was maintained in the laboratory by holding 

 naturally infected fish in troughs . Infections 

 in both stock fish and experimentally infected 

 fish were determined most easily by examina- 

 tion of liver and kidney smears In most 



Parenteral - - The first attempts to transmit 

 the disease in our laboratory were 

 made by intxaperitoreal inoculation 

 into young fish which came from 

 stocks in which the disease had never 

 been known. The fish used were 

 rainbow trout ( Salmo gairdneri) . 

 silver salmon ( Oncorh^mchus kisutch) , 

 sockeye salmon (O^. nerka), and squaw- 

 fish ( Ptychoche jj^is oregonensis) . 



1 



